Some types of AC-AC frequency converters may comprise a plurality of neutral point clamped converter units. These converter units may be connected in parallel for providing the phases of the converter and/or in series for increasing the output voltage and the number of voltage levels of a phase.
Examples for such converters may be found in CN 202197222 U and in WO2006/000111 A1.
Each converter unit of the converter may have a DC link with a DC (Direct Current) capacitor to accumulate energy, which is used for producing the output voltage through switching states of an inverter interconnecting the DC link with the respective phase output. Due to reasons of security, the DC capacitors usually have to be discharged during maintenance or when the converter is stopped.
One possible discharging method is done via a high voltage relay and an arrangement of discharging resistors. However, this method requires additional hardware elements, which may increase production and maintenance costs of the converter.
Some other methods for different converter topologies are presented in WO2012/013245 A1 and US2008/0278117 A1.
EP 2 541 746 A2 shows that a DC link capacitor may be discharged into a grid, which may be interconnected via chokes with a converter. In EP 2 541 746 A2 an NPC converter is mentioned.
US 2005/0231171 A1, JP 2009 017750 A and DE 42 35 138 A1 show two-level converters with a DC link capacitor that may be discharged into filters or a load.